The present invention relates to an ignition circuit for internal combustion engines, and more particularly it relates to an improvement on the induction dicharge type non-contact ignition circuit for internal combustion engines wherein the primary current passing through the primary winding of the magneto dynamo ignition coil is abruptly interrupted by the cutting-off of the power transistor in order to cause spark discharges in the spark plug which is connected to the secondary winding of said ignition coil.
Various types of induction discharge type non-contact ignition circuit for internal combustion engines utilizing transistor circuits have been developed and put into practical use. Although these circuits require no maintenance and/or inspection of the contact breaker and have been improved in respect of its durability, they are defective in that the stable spark discharge cannot be achieved unless the revolution rate of the magneto dynamo exceeds a certain rate since these circuits are constructed in such a way that the primary current is cut off when the same reaches a predetermined value.
More particularly, the internal combustion engines for such tools as chain saws are driven by manually pulling the recoiling rope for starting the same. The ignition circuit is therefore so set that when the revolution rate of about 750 rpm is obtained, the spark discharge sufficient for activation can be obtained. If the revolution rate is set too low, the ignition performance at the time of high speed driving becomes insufficient, and this naturally sets the lower limit. In case of the conventional ignition circuit of a contact breaker type, it was possible to confirm the spark discharge by lightly pulling the recoiling rope to check the functions of the ignition plugs. In case of the transistor type ignition circuits, on the other hand, the spark discharge cannot be performed when the recoiling rope is pulled at a strength which would normally give the revolution rate of about 300 to 500 rpm. It is necessary to pull the same at a still greater strength, causing confusion for operators who are used to handling the conventional contact breaker type circuits.